The present invention relates to a method for improving the acoustic sidetone suppression in hands-free telephones, having a level discriminator and a plurality of adaptive echo compensation filters of which each processes a subband.
In the case of a hands-free telephone it is mandatory to suppress the signals of the remote subscriber which are emitted by the loudspeaker and therefore received again by the microphone, since otherwise unpleasant echoes disturb the connection. Until now, it has been customary for the purpose of suppressing these echoes, that is to say for the purpose of acoustic sidetone suppression, to provide a level discriminator which strongly damps the transmitting or receiving path as a function of the call situation. However, this renders duplex (full duplex mode) virtually impossible.
It has already been attempted using the prior art to date to make an adequate sidetone suppression available despite acceptable duplex mode properties. For this purpose, a frequency-selective, controllable echo suppression is provided in addition to the level discriminator. Such methods are described, for example, in the NEC advertizing brochure xe2x80x9cREFLEXION Acoustic Echo Canceller on the xcexcPD7701x Familyxe2x80x9d, 1996, or in a description of the Motorola DSP5600x digital processor by M. Knox, P. Abbott, C. Cox, titled xe2x80x9cA Highly Integrated H.320 Audiosubsystem Using The Motorola DSP5600x Digital Processorxe2x80x9d.
In the article titled xe2x80x9cWideband Acoustic Echo Canceller With Two Filter Structurexe2x80x9d by W. Armbruister in Signal Processing Theories and Applications, Brussels, volume 3, No. CONF. 6, Aug. 24, 1992, pages 1611-1614, there is a description of a device for echo compensation in the case of video conferences which operates with an adaptive and a non-adaptive filter for linear compensation.
Published, European Patent Application EP 0 739 102 A2 describes a method for echo compensation in the case of which the received signal is split into a multiplicity of subbands for which the echo signal is estimated in each case.
Such echo suppression methods function satisfactorily in normal rooms. In the case of the use of hands-free telephones in motor vehicles, the detection of duplexxe2x80x94independently of the interior acousticsxe2x80x94becomes conspicuously more difficult than in office rooms. In particular, it is exceptionally difficult in motor vehicles to distinguish abrupt changes in the interior acoustics, for example, owing to movements of the vehicle occupants, from the duplex.
It is accordingly an object of the invention to provide a method for improving acoustic sidetone suppression in hands-free telephones that overcomes the disadvantages of the prior art methods of this general type, in which it is possible even in motor vehicles to detect a clear distinction between duplex and abrupt changes in the interior acoustics, and to take account of the distinction when controlling the hands-free telephone.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for improving acoustic sidetone suppression, which includes providing a hands-free telephone having a level discriminator and a plurality of adaptive echo compensation filters each processing a subband. A further adaptive filter of a different order is connected in parallel with an adaptive echo compensation filter of the adaptive echo compensation filters in at least one half band. Spatial changes are then detected using a correlation analysis and a power evaluation of an output of the further adaptive filter.
According to the invention, the further adaptive filter (shadow filter) of a lower order is connected in parallel with the adaptive echo compensation filter in at least one subband. Spatial changes can then be detected by the combination of a power evaluation of the two residual echo powers and a correlation analysis of the estimated and the measured microphone signals.
Use can preferably be made in this case of a plurality of different sampling rates. The computational outlay can be reduced thereby.
It is also preferred for the further adaptive filter to be of a substantially lower order.
The echo compensation is preferably implemented by a filter bank in frequency subbands.
Both power evaluations of the competing adaptive filters, and correlation-based analyses are preferably used for controlling the adaptation or step size.
It is likewise preferred for power transmission factors to be estimated in subbands in order to determine the step size.
It is likewise preferred for the echo compensation filters, to supply estimated values for the return loss introduced by them, since these estimated values can preferably be used for controlling the attenuation of the level discriminator. Consequently, the attenuation to be introduced by the level discriminator can be further reduced, and thus the call quality can be further improved in the case of duplex.
It is additionally preferred to detect the simultaneous activity of both call participants (duplex). It is then possible, for example, to reduce the overall attenuation of the level discriminator in the duplex case, in order further to improve the full duplex mode of the hands-free telephone.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for improving acoustic sidetone suppression in hands-free telephones, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.